A walking assist device which assists walking motion of a user by applying torque to the user's knee joint has been developed. Japan Patent Application Publication No. 2009-207840 discloses one example of such a walking assist device. The typical walking assist device may have a multi-link mechanical structure that is to be attached along the user's leg. One link of the multi-link structure is to be attached to the user's upper leg. Other one link is to be attached to the user's lower leg. The upper link and the lower link are connected by the rotary joint. When the user wears the walking assist device, the rotary joint is coaxially aligned with the knee joint. According to such configuration, the lower link is able to swing while keeping parallel to user's lower leg. This walking assist device is also provided with an actuator that swings the lower link. By the output power of the actuator, the plurality of links emulates the ideal walking motion of the leg and thus, the walking assist device guides the user's leg motion. The multi-link type walking assist device provided with the actuator may be commonly called a “robot suit”.
The motion of the leg during walk is basically defined by the swing of the upper leg around the hip joint pitch axis and the swing of the lower log around the knee joint pitch axis. In other words, the leg motion during walk is defined by the time-dependent changes of the hip joint angle (around the pitch axis) and the knee joint angle (around the pitch axis). The walking assist device stores the target rotation angle time series data (the target trajectory) that simulates the time-dependent changes of the hip joint angle and the knee joint angle during walk, and the device drives the rotary joints corresponding to the knee and hip joints of the user so that each follows its target trajectory. It is noted that the walking assist device may store the predetermined target trajectories, or the device may generate, in real time, the target trajectories in response to the characteristics of the user's walking motion such as walking speed and/or stride length. Also, it is noted that “the joint angle” of the user's joint and “the joint angle” of the rotary joint of the walking assist device are not distinguished hereinafter for simplification of the explanation. Thus, the target angle (target trajectory) of the rotary joint corresponding to the hip joint may be referred to as the target angle (target trajectory) of the hip joint, and the target angle (target trajectory) of the rotary joint corresponding to the knee joint may be referred to as the target angle (target trajectory) of the knee joint, or referred to as the target knee joint angle. Furthermore, the hip joint angle corresponds to the swing angle of the upper leg, and the knee joint angle corresponds to the swing angle of the lower leg. Thus, the target knee joint angle may be referred to as the target swing angle of the lower link.